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Transcript of Thomas J. Preston, Maitreya Dutta, Brian J. Esselman, Michael A. Shaloski, Robert J. M C Mahon, and...
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Thomas J. Preston, Maitreya Dutta, Brian J. Esselman, Michael A. Shaloski, Robert J. MCMahon, and F. Fleming Crim
UW-Madison
Aimable Kalume, Lisa George, and Scott A. Reid
University of Marquette
Isomerization Between CH2ClI and CH2Cl-I in Cryogenic Matrices Studied on Ultrafast
Timescales
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Isomerization of HalomethanesFirst identified in
solid matrices with frequency-domain spectroscopy*
Return to parent following electronic excitation
Stable in cryogenic solids
Transient in liquids
*Maier…Hess, JACS, 112, 5117 (1990).
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Experiment
Frequency Domain Time Domain
500:1 Pulsed deposition
Tunable, Nd:YAG nanosecond dye laser
Static, difference spectroscopy
220:1 Continuous deposition
Ultrafast, Ti:Sapphire laser
Transient absorption spectroscopy
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Potential Energy Sketch
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Transient Absorption - Photolysis
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Transient Absorption - Probe
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Transient Absorption - Recovery
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Spectroscopy of Parent and Isomer
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Spectroscopy of Parent and Isomer
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Pump and Probe Wavelengths
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Probe Vibrational Energy with Electronic Transition
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Probe Vibrational Energy with Electronic Transition
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Probe Vibrational Energy with Electronic Transition
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Isomer Absorption In Ar, N2, CH4
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Transient Absorption: CH4
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Transient Absorption: CH4
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Transient Absorption: CH4
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Transient Absorption: CH4
Rise time, t1 / ps
1.1(2)
1.4(1)
2.3(3)
3.1(3)
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Transient Absorption: CH4
Rise or fall time, t2 / ps
42(7)
276(122)
49(13)
37(5)
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Potential – 1D
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Isomer Formation and Relaxation
t1: 1 ps
t2: 40 ps
MP2/Sadlej-pVTZ
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Trends Same in CH4, N2, and Ar
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Complete PictureVibrationally excited isomer forms in about
1 psVibrational relaxation occurs in about 40 psBoth of these timescales are similar in
liquid and super-critical fluids
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Unanswered QuestionsWhat is the detailed process of formation
(t1)?Formation time is 2x faster in N2 and CH4
than in ArWhat is the detailed process of cooling (t2)?
Rates are indiscriminate of matrix complexityRates similar among solids, liquids, super-
critical fluidsOpportunity for MD simulations
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AcknowledgementsScott Reid and group at Marquette
Static spectroscopyCalculations
Bob McMahon and group at UW-MadisonMatrix isolation apparatusCalculations
Fleming Crim and group at UW-Madison
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End of slide show, click to exit.
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Thomas J. Preston, Maitreya Dutta, Brian J. Esselman, Michael A. Shaloski, Robert J. McMahon, and F. Fleming Crim
UW-Madison
Aimable Kalume, Lisa George, and Scott A. ReidUniversity of Marquette
Isomerization of CH2Cl-I to CH2ClI in Cryogenic Matrices:
A Study on Ultrafast Timescale
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Recap: Formation of the Isomer
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Explore Potential Energy Surface
a
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Setup
Probe
Preparation
Pump
Destruction
Wall-Clock Time
Isom
er
Popu
lati
on
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Probe Isomer or Parent?
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Probe Isomer or Parent?
Prep.
Pump Probe
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Destruction of Isomer
lpump = 400 nm
lprobe = 400 nm
lpump = 800 nm
lprobe = 400 nm
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IR Pump – C-H Stretch Overtone
Loss to Parent
VibrationalRelaxation
6000 cm-1
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IR Pump – C-H Stretch Fundamental
3000 cm-1
VibrationalRelaxation
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Vibrational Relaxation
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Vibrational Relaxation
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Vibrational Relaxation
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Vibrational Relaxation
Excite C-H stretchEnergy flows into
Franck-Condon mode (t1)
Energy flows into surroundings (t2)
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Infrared Excitation100 mm-thick samples10-20% conversion to isomerSmall absorption at C-H overtoneExceptionally large cross section at C-H
fundamental
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AcknowledgementsScott Reid and group at Marquette
Static spectroscopyCalculations
Bob McMahon and group at UW-MadisonMatrix isolation apparatusCalculations
Fleming Crim and group at UW-Madison